Fuel cell systems capable of high-efficiency small-scale power generation have been and are being developed as a distributed power generation system having high energy utilization efficiency, because they have a system for utilizing heat energy generated during power generation which is easy to construct.
Fuel cell systems have a fuel cell as the main body of the power generation section. In this fuel cell, the chemical energy of fuel gas and oxidizing gas is directly converted into electric energy through a specified electrochemical reaction. Therefore, fuel cell systems are configured to supply fuel gas and oxidizing gas to the fuel cell during power generating operation. In the fuel cell, the specified electrochemical reaction, which uses the supplied fuel gas and oxidizing gas, proceeds so that electric energy is generated. The electric energy generated in the fuel cell is supplied from the fuel cell system to the load. Generally, fuel cell systems have a reformer and a blower. In the reformer, hydrogen-rich fuel gas is generated through the steam reforming reaction that uses water and raw material such as natural gas. This fuel gas is supplied to the fuel cell as a fuel for power generation. The steam reforming reaction proceeds with a reforming catalyst provided in the reformer being burnt by, e.g., a combustion burner. The blower draws air from the atmosphere. This air is supplied to the fuel cell as the oxidizing gas for power generation.
In a known fuel cell system, the supply of the raw material such as natural gas to the reformer is stopped when stopping power generating operation. Thereby, the supply of the fuel gas from the reformer to the fuel cell is stopped so that the progress of the electrochemical reaction within the fuel cell stops and, in consequence, the supply of electric power from the fuel cell system to the load stops. When the supply of the raw material to the reformer is stopped, the fuel gas generated before the stop stays within the fuel cell and its neighboring part during a period of time when the power generation is stopped. In this case, if air comes from the combustion burner opened to the atmosphere and gets mixed in with the dwelling fuel gas owing to natural convection, the hydrogen contained in the fuel gas is rapidly oxidized by oxygen contained in air so that the reaction heat accompanying the oxidizing reaction may damage the fuel cell system.
Therefore, the known fuel cell system is configured such that, in order to prevent the fuel gas from staying within the fuel cell system, inert gas such as nitrogen gas is fed to the path in which the fuel gas is staying during a power generation stop period to force out the fuel gas which is in turn combusted by a combustion burner. With this arrangement, the stay of the fuel gas within the fuel cell during the power generation stop period can be prevented so that rapid oxidation of the hydrogen contained in the fuel gas can be avoided. As a result, a fuel cell system, which assures security, can be obtained.
However, in the known fuel cell system, an inert gas feeding means such as a nitrogen gas cylinder has to be installed within or near the fuel cell system to replace the dwelling fuel gas with the inert gas such as nitrogen gas. Therefore, the known fuel cell system is large in size and difficult to use as a fixed-type distributed power generation system for household use or a power source for electric vehicles. In addition, the means for feeding inert gas such as nitrogen gas has to be provided in addition to the existing components, which increases the initial cost of the fuel cell system. Furthermore, the known fuel cell system is required to periodically replace or replenish the inert gas feeding means such as a nitrogen gas cylinder, so that the running cost of the fuel cell system increases.
In the known fuel cell system, the fuel gas containing high concentrations of carbon monoxide is fed from the reformer to the fuel cell just after starting power generating operation. The reason for this is that carbon monoxide contained in the fuel gas is not thoroughly removed because the operating temperature of the reformer has not reached a specified value at a start of power generating operation. Therefore, if the fuel gas containing high concentrations of carbon monoxide is fed to, for example, a solid polymer electrolyte fuel cell, the catalyst of the fuel electrode of the solid polymer electrolyte fuel cell is contaminated (poisoned) with the carbon monoxide supplied. The poisoning of the catalyst of the fuel electrode significantly hampers the progress of the electrochemical reaction within the fuel cell. Therefore, the known fuel cell system has presented the problem that the power generation performance of the fuel cell deteriorates depending on the number of stops and starts of power generating operation.
As an attempt to solve the above problems, there has been proposed a fuel cell system that is usable for household purposes and electric vehicles and the catalyst of which is unsusceptible to poisoning (e.g., Patent Document 1). According to this system, feeding of the fuel gas to the fuel cell is stopped just after starting power generating operation and the fuel gas serving as a raw material is injected into the fuel cell as a displacement gas after stopping power generating operation.
The above proposed fuel cell system has a reformer for generating hydrogen-rich fuel gas from a raw material containing, as a chief component, a compound of carbon and hydrogen; a fuel gas feed passage for feeding the fuel gas from the reformer to a fuel cell; an off gas feed passage for feeding the fuel gas, which has been discharged from the fuel cell without being used for power generation (hereinafter referred to as “off gas”), to a combustion burner of the reformer; and a first bypass passage provided between the fuel gas feed passage and the off gas feed passage, for switching the destination of the fuel gas from the fuel cell to the combustion burner of the reformer. In addition, the fuel cell system includes a raw material feeder for feeding a raw material to the reformer to generate the fuel gas and a second bypass passage that extends from the raw material feeder to the fuel cell bypassing the reformer to directly send the raw material to the fuel cell.
In the proposed fuel cell system, just after starting power generating operation, the fuel gas containing high concentrations of carbon monoxide and generated in the reformer is fed to the combustion burner of the reformer by way of the first bypass passage. In the combustion burner, the fuel gas is combusted to heat the reforming catalyst. After the temperature of the reforming catalyst in the reformer has reached a specified temperature after starting power generating operation, the fuel gas generated in the reformer is fed to the fuel cell via the fuel gas feed passage. In the fuel cell, the fuel gas is used as a fuel for power generation. The off gas discharged from the fuel cell is fed to the combustion burner of the reformer via the off gas feed passage. In the combustion burner, the off gas is combusted for heating the reforming catalyst.
In the proposed fuel cell system, after stopping the power generating operation of the fuel cell system, a raw material is injected as a displacement gas from the raw material feeder into a fuel gas flow path of the fuel cell through a second bypass passage. Thereby, the inside and neighboring area of the fuel cell are sealed off by the raw material such as natural gas in place of the inert gas such as nitrogen gas over a period of time when the power generating operation of the fuel cell system is stopped.
According to the above fuel cell system, since the raw material is injected as a displacement gas into the fuel cell from the raw material feeder that is originally provided, after stopping power generating operation, it is no longer necessary to dispose an inert gas feeding means such as a nitrogen gas cylinder within or in the neighborhood of the fuel cell system. Accordingly, the fuel cell system is not increased in size and therefore can be used as a fixed-type distributed power generation system for household use or a power source for electric vehicles. In addition, since there is no need to provide an inert gas feeding means such as a nitrogen gas cylinder in addition to the original components, the initial cost of the fuel cell system can be kept low. Furthermore, there is no need to periodically replace an inert gas feeding means such as a nitrogen gas cylinder, which leads to a reduction in the running const of the fuel cell system.
The raw material such as natural gas injected from the raw material feeder into the fuel cell is more chemically stable compared to the hydrogen contained in the fuel gas. Therefore, no rapid oxidation reaction will proceed even if air is mixed in with the raw material such as natural gas dwelling within the fuel cell during a power generation stop period. Therefore, the fuel cell system can be effectively prevented from being damaged by the reaction heat of oxidation reaction by injecting the raw material such as natural gas into the fuel cell. As a result, the proposed fuel cell system can assure security during the power generation stop period.
Further, according to the proposed fuel cell system, fuel gas containing high concentrations of carbon monoxide is not supplied to the fuel cell just after starting power generating operation, but fuel gas is fed from the reformer to the fuel cell after the temperature of the reforming catalyst of the reformer reaches a specified value and fuel gas containing a sufficiently reduced concentration of carbon monoxide is generated. Therefore, the poisoning of the catalyst of the fuel electrode in the solid polymer electrolyte fuel cell be prevented. Since the factor for impeding the progress of the electrochemical reaction within the fuel cell is thus eliminated, it is possible to solve the problem that the power generating performance of the fuel cell deteriorates depending on the number of stops and stars of power generating operation.
Patent Document 1: Japanese Patent Application Laid-Open Publication No. 2003-229149